It's pretty much accidental that that ever worked. There are cases where mixing positional and keyword arguments will given incorrect results in the previous code and even more so now (with optional groups). So, ok, there's an inadvertent backwards incompatibility for something that only worked by accident. It's simple to work around: pass in either just positional arguments or just keyword arguments. Both are easy enough to do.

Somebody will update the BackwardsIncompatibleChanges page at some point and we can add a docs note. For now, the error message is clear enough about what needs to be changed in anybody's code that was relying on this.